1. Technical Field
The present invention relates to a conductive pattern formation ink, a conductive pattern and a wiring substrate, and more specially relates to a conductive pattern formation ink, a conductive pattern formed by the conductive pattern formation ink and a wiring substrate provided with the conductive pattern.
2. Related Art
A wiring for use in an electronic circuit or an integrated circuit is produced using a photolithography method or the like.
In this photolithography method, a photosensitive material, which is called a resist, is applied on a substrate pre-coated with a conductive film, light is irradiated on the resist at a pattern corresponding to a wiring to be formed, the resist irradiated is developed to form a resist pattern, and the conductive film is etched using the resist pattern to obtain the wiring on the substrate.
The photolithography method requires use of bulky equipments such as a vacuum apparatus and the like and a complex process. In the photolithography method, efficiency of using a material is as low as about several percentages, which means that there is no choice but to waste the material for the most part. This leads to an increase in production costs.
Meanwhile, there has been proposed a method of forming a conductive pattern (wiring), which makes use of what is called an ink jet method, i.e., a liquid droplet ejecting method by which a liquid material is ejected in the form of liquid droplets from a liquid ejection head (see, e.g., U.S. Pat. No. 5,132,248).
With this method, a conductive pattern formation ink, in which conductive particulates are dispersed, is directly applied on a substrate according to a desired pattern, and the ink thus applied is transformed into a conductive pattern by subsequently evaporating a solvent through a heat treatment or laser irradiation. This method enjoys merits in that it requires no photolithography, bitterly simplifies a process and reduces a quantity of raw materials used.
However, a conductive pattern produced by a conventional conductive pattern formation ink suffers from a drawback in that cracks are likely to be generated in the conductive pattern during a course of evaporating the solvent, which may lead to an increase in specific resistance of the conductive pattern and may result in disconnection of the conductive pattern. In particular, generation of the cracks becomes severe as the conductive pattern grows thicker.
Causes of generating such cracks are presumed to be sudden contraction in volume of the conductive pattern during evaporation of the solvent, contraction in volume of the conductive pattern arising from breakaway of a dispersant adhering to the conductive particulates, enlargement of pores of the conductive pattern attributable to grain growth of silver particulates caused by heating during evaporation of the solvent, and so forth.
Furthermore, if the pores of the conductive pattern are enlarged due to the grain growth of the silver particulates and come up to a surface of the conductive pattern, the conductive pattern exhibits reduction in its surface planarity. This poses a problem in that what is called a skin effect is not attained, consequently reducing high-frequency characteristics of the conductive pattern.
When a relatively thick conductive pattern is formed by the ink jet method, it is sometimes a case that a conductive pattern formation ink is overlappingly applied on a substrate. In this case, a previously applied layer of ink is dried first (referred to as a preliminary drying step) and then the next layer of ink is applied in an effort to prevent disconnection or warp in shape of the conductive pattern.
In the conductive pattern formation method noted just above, it is often a case that the finished conductive pattern becomes a laminated structure because the conductive pattern formation ink applying step and the preliminary drying step are repeated alternately. With the conductive pattern of this laminated structure, interlayer specific resistance may sometimes be increased and specific resistance of the conductive pattern as a whole may become higher.